2-Acetyl-1-pyrroline (2AP) was first identified in cooked rice (Buttery, et al. (1982) Chem. Ind. 12:58-959) and was regarded as the most powerful odorant among all rice volatiles identified (Buttery, et. al. (1983) J. Agric. Food Chem. 36:1006-1009). 2AP, with an odor threshold of 0.1 μg/L in water (Buttery, et al. (1982) supra), dominates its homologues 6-acetyl-1,2,3,4-tetrahydropyridine (ATHP, including tautomer 6-acetyl-2,3,4,5-tetrahydropyridine), 2-propionyl-1-pyrroline (2PP), and 2-acetyl-2-thiazoline (2AT), all of which possess roasted, cracker-like, and popcorn-like aroma notes and are also important flavorants in numerous food products (Adams, et al. (2006) Chem. Rev. 106:299-2319).
The specialty fragrant or aromatic rice varieties contain high levels of 2AP. In an effort to boost the production of 2AP, the gene linked to the accumulation of 2AP in fragrant rice has been identified (Bradbury, et al. (2005) Plant Biol. J. 3:363-370) and transferred into non-aromatic rice varieties, thereby enabling the accumulation of greater levels of 2AP (U.S. Pat. No. 7,319,181). 2AP is of commercial interest to the flavor industry because of its pleasant and characteristic flavor. However, the highly unstable nature of the compound has hindered its widespread commercial use. In popcorn, 2AP decreases by about 80% during room temperature storage for one week in polyethylene bags (Schieberle (1995) J. Agric. Food Chem. 43:2442-2448). In raw fragrant rice, the concentration of 2AP was shown to diminish by about half its original content after 3 months (Widjaja, et al. (1996) J. Sci. Food Agric. 71:218-224). The instability of 2AP has been postulated to occur via a polymerization reaction (Buttery, et al. (1982) supra). The cyclic imine of 1-pyrroline was shown to form trimers during work-up, which then undergo condensation with neighboring molecules to form polymeric products (Fuhlhage, et al. (1958) J. Am. Chem. Soc. 80:6249-6254).
Some efforts have been made to increase the stability of 2AP. For example, a process for encapsulation of 2AP with β-cyclodextrin (β-CD) has been described (U.S. Pat. No. 5,512,290). However, 2AP (10% loading) was decomposed by 91% after 13 days of storage at 20° C. At a loading of 1%, 2AP decomposed by 99% after 110 days of storage. Additional efforts to stabilize 2AP have been suggested through encapsulation, where gum acacia and/or starch are applied to form a stabilized dry powder (US 2006/0147596). However, no proven stability was demonstrated. A crude extract from pandan leaves, a natural source of 2AP, has also been mixed with β-CD to form a powder (US 2012/0213904). However, storage stability data were not reported to demonstrate the effectiveness of this approach. In additional study, 2AP was maintained at a level of 70% after 72 days of storage at ambient temperature when encapsulated at very low loading (0.003%) in a gum acacia/maltodextrin matrix (Apintanapong, et al. (2003) Int. J. Food Sci. Tech. 38:95-102).
U.S. Pat. No. 3,373,177 describes metal complexes of β-imino-ketones, which are stable and can be used in fungicides and pesticides, particularly those metals having known toxic effects such as copper, mercury, beryllium and silver. Similarly, U.S. Pat. No. 5,227,156 describes the use of zinc compounds to stabilize a tiazolinone preservative in an anti-dandruff shampoo.
Previous attempts to stabilize 2AP suffer to some extent from at least one of the following deficiencies: 1) stability of 2AP was either not mentioned or was insufficiently studied; 2) low loadings of 2AP were used; and/or 3) 2AP was only stable during storage at low temperature. Therefore, alternative approaches to stabilizing 2AP are needed in the art.